The acceptance of high pressure sodium lamps is steadily increasing since their introduction in 1966. This type of light source is more efficient than incandescent, fluorescent, or mercury lamps, and shows considerably improved color rendition over low pressure sodium lamps. Due to the current emphasis on conservation of energy, a continued growth of the market for HPS lamps is expected for adequate illumination in industrial, public, commercial, and now even in consumer applications with reduced energy consumption.
The main part of the high pressure sodium HPS lamp is the ceramic arc tube, usually fabricated from high density polycrystalline alumina, which contains the gas discharge. In addition, high light transmission is required to transmit the visible energy produced by the discharge. With the lamp operating at high temperatures and high pressures and containing a sodium amalgam fill only a few materials are suitable to contain the sodium. Even by the use of ceramics such as polycrystalline aluminum (PCA), small amounts of sodium escape from the arc tube by diffusion through the polycrystalline wall structure and are deposited on the wall of the evacuated outer jacket of the lamp, causing reduced transmission of the glass and decreased light output with time. Due to the statistical nature of grain growth, small grain-boundary defects do occur, which can be an additional route for sodium loss from the arc tube.
Chemical analysis of the inner wall of HPS lamp jackets confirmed the presence of a high sodium content. Therefore, it would be desirous to prevent sodium migration into and through the polycrystalline arc tube wall.